WASHINGTON (CNN) -- Waterboarding is necessary though probably not legal, CIA Director Michael Hayden told Congress Thursday as Attorney General Michael Mukasey said he would not open a criminal investigation into the CIA's use of the technique.

CIA Director Michael Hayden said waterboarding is needed to interrogate but it probably isn't legal.

Strapping a person to a surface, covering their face with cloth and pouring water on their face to imitate the sensation of drowning could be used if "an unlawful combatant is possessing information that would help us prevent catastrophic loss of life of Americans or their allies," said Hayden.

"In my own view, the view of my lawyers and the Department of Justice, it is not certain that that technique would be considered lawful under current statute," he told the House Intelligence Committee after publicly disclosing that the CIA had used waterboarding on three of the enemy combatants.

He explained that the method was used because of "mis-shaped and misformed" political discussion about waterboarding. Watch Hayden describe current CIA policy on waterboarding »

Hayden reiterated that the technique is not part of the interrogation program now and that the waterboarding techniques, when they were used in the 2002 and 2003, were limited to three top al Qaeda suspects

Also Thursday, Attorney General Michael Mukasey told lawmakers he will not open a criminal investigation into the CIA's use of waterboarding on terror suspects.

House Judiciary Committee Chairman John Conyers asked Mukasey bluntly whether he was starting a criminal investigation since Hayden confirmed the use of waterboarding.

"No, I am not, for this reason: Whatever was done as part of a CIA program at the time that it was done was the subject of a Department of Justice opinion through the Office of Legal Counsel and was found to be permissible under the law as it existed then," he said.

Mukasey said opening an investigation would send a message that Justice Department opinions are subject to change.

"Essentially it would tell people, 'You rely on a Justice Department opinion as part of a program, then you will be subject to criminal investigations ... if the tenure of the person who wrote the opinion changes or indeed the political winds change,'" he said. "And that's not something that I think would be appropriate and it's not something I will do."

Conyers, D-Michigan, and Mukasey argued over whether the Justice Department will provide documents on the waterboarding opinion to the committee.

Mukasey refused, saying the documents are highly classified and that he had already said he is not going to open an investigation.

Conyers and other House Democrats then called for the criminal investigation. E-mail to a friend

CNN's Pam Benson contributed to this report.

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